Controlling Simulation Speed and Accuracy

Convergence is defined as the ability to obtain a solution to a set of circuit equations within a given tolerance criteria and number of iterations. In numerical circuit simulation, the designer specifies a relative and absolute accuracy for the circuit solution and the simulator iteration algorithm attempts to converge to a solution that is within these set tolerances. In many cases, the speed of reaching a solution also is of primary interest to the designer, particularly for preliminary design trials, and some accuracy is willingly sacrificed.

Simulation Speed

Star-Hspice can substantially reduce the computer time needed to solve complex problems. The following user options alter internal algorithms to increase simulation efficiency.

Simulation Accuracy

Star-Hspice is shipped with control option default values that aim for superior accuracy while delivering good performance in simulation time. The control options and their default settings to maximize accuracy are:

NOTE: BYPASS is only turned on (set to 1) when DVDT = 4. For other DVDT settings, BYPASS is off (0). SLOPETOL is set to 0.75 when DVDT = 4 and LVLTIM = 1. For all other values of DVDT or LVLTIM, SLOPETOL defaults to 0.5.

Timestep Control for Accuracy

The DELMAX control option also affects simulation accuracy. DELMAX specifies the maximum allowed timestep size. If DELMAX is not set in an .OPTIONS statement, Star-Hspice computes a DELMAX value. Factors that determine the computed DELMAX value are:

.OPTIONS RMAX and FS

Breakpoint locations for a PWL source

Breakpoint locations for a PULSE source

Smallest period for a SIN source

Smallest delay for a transmission line component

Smallest ideal delay for a transmission line component

TSTEP value in a .TRAN analysis

Number of points in an FFT analysis

The FS and RMAX control options provide some user control over the DELMAX value. The FS option, which defaults to 0.25, scales the breakpoint interval in the DELMAX calculation. The RMAX option, which defaults to 5 if DVDT = 4 and LVLTIM = 1, scales the TSTEP (timestep) size in the DELMAX calculation.

For circuits that contain oscillators or ideal delay elements, an .OPTIONS statement should be used to set DELMAX to one-hundredth of the period or less.

The ACCURATE control option tightens the simulation options to give the most accurate set of simulation algorithms and tolerances. When ACCURATE is set to 1, it sets the following control options:

Algebraic models that describe parasitic interconnect capacitances as a function of the width of the transistor. The wire model extension of the resistor can model the metal, diffusion, or poly interconnects to preserve the relationship between the physical layout and electrical property.

MOS model parameter ACM that calculates defaults for source and drain junction parasitics. Star-Hspice uses ACM equations to calculate the size of the bottom wall, the length of the sidewall diodes, and the length of a lightly doped structure. SPICE defaults with no calculation of the junction diode. Specify AD, AS, PD, PS, NRD, NRS to override the default calculations.

MOS model parameter CAPOP = 4 that models the most advanced charge conservation, non-reciprocal gate capacitances. The gate capacitors and overlaps are calculated from the IDS model for LEVEL 49 or 53, however, the CAPOP parameter is ignored, model parameter CAPMOD with reasonable value should be used instead.

Guidelines for Choosing Accuracy Options

Use the ACCURATE option for

Analog or mixed signal circuits

Circuits with long time constants, such as RC networks

Circuits with ground bounce

Use the default options (DVDT = 4) for

Digital CMOS

CMOS cell characterization

Circuits with fast moving edges (short rise and fall times)

For ideal delay elements, use one of the following:

ACCURATE

DVDT = 3

DVDT = 4, and, if the minimum pulse width of any signal is less than the minimum ideal delay, set DELMAX to a value smaller than the minimum pulse width.